1. Field of the Invention
The invention relates to excavator drive systems that control excavator propulsion and bucket motion, and in particular motor transfer switches in such drive systems that selectively couple and decouple drive system motors from shared power sources.
2. Description of the Prior Art
Excavator systems, such as electric shovels and drag lines, are critical high power-consuming equipment in the mining industry. As shown in FIGS. 1 and 2, a known shovel system 10 includes a chassis to which are mounted a pair of tracks 12, 14, a boom 16, a dipper arm 18 and a bucket 20. The excavator 10 has a drive system 30 that enables the designated individual motions referred to as hoist, swing, crowd and propel. Those motions are typically powered by multi-phase AC asynchronous motors 40, 42, 44, 46 that are fed by active front end rectified inverters 32 including IGBT vector control. The inverters 32 reduce the harmonics associated with rectification and provide reactive power support at the shovel's point of common coupling (PCC).
The known excavator drive system 30, shown in FIG. 2, has a propel motion system comprising two independently controlled motors 44, 46 powering respective tracks 12, 14. For descriptive simplicity only a single phase of the multi-phase system is shown, it being readily apparent to those skilled in the art that the other phases have similar construction, function and operation. Transfer switches 34 allow sharing of power source inverters 32 between pairs of crowd and propel 1 motion motors 40, 44 and hoist and propel 2 motion motors 42, 46. Each transfer switch 34 is controlled by a programmable logic controller (PLC) 38; they communicate with each other via communications bus 39 or other known communications pathway.
Typical known industrial transfer switches 34 have a motor module 35 that drives multiple switch modules 36 (often known multi-phase motor contactors) through an external mechanical linkage. The switch modules 36 typically are single pole double throw or double pole, double throw motor contactors having external mechanical “on”/“off” switches 37. The motor module 35 mechanically interfaces with the switch module external mechanical switches 37 and functionally enables remote mechanical actuation under control of a programmable logic controller (PLC) 38. As shown in FIG. 2 a representative motor module 35 includes a reversable motor 35A driving a mechanical linkage (here pinion 35B engaging rack 35C interface with on/off switches 37).
The known commercialized motor module 35, being a mechanical device, typically needs greater than 2 seconds to perform a motor transfer, due to system response phase lag. Quicker transfer time is desired to increase excavator productivity. As an example, if in a typical operational hour there are 5 transfers between propel and digging modes and each transfer expends 3 seconds, that results in a loss of 30 seconds per hour. The expended time presents an opportunity to gain productivity of an additional digging cycle with an expensive piece of earthmoving equipment.
In the event of loss of power to the transfer switch 34, some of the known switch module contactors 36 lose power and default to a driven motor “off” condition, in which case the excavator ceases motion. If prior to transfer switch 34 power loss the excavator 10 is performing a translation or digging motion the operation ceases, even though the excavator operator did not need to transfer motor power. If the prior state of motor connection was preserved after a transfer switch 34 power failure, as is possible with some known commercialized devices, the operator could continue excavator operation in that prior connection status mode.
Thus, a need exists in the art for an excavator, including an excavator drive system that is capable of transferring electric power from one drive motor to another drive motor without mechanical external transfer linkages and auxiliary motor drives that typically have long transfer time lags from initiation of a transfer command to completion of the power transfer.
Another need exists in the art for an excavator, including an excavator drive system, that does not cause disruption of power to drive system motors due to a transmit switch power failure, so that an operator can continue to use the excavator drive system in the manner preceding the power failure.